1. Field of the Invention
The invention relates to a frequency mixer circuit, and more particularly to a balanced frequency mixer circuit.
2. Description of the Related Art
FIG. 1 shows a conventional Gilbert cell mixer including a frequency mixer stage 10, a transconductance stage 20 and resistors R1, R2.
The frequency mixer stage 10 includes transistors Q1, Q2, Q3 and Q4. The gate terminals of the transistors Q1, Q2 cooperate with the gate terminals of the transistors Q3, Q4 to receive a differential local oscillator voltage signal LO from a local oscillator (not shown).
The transconductance stage 20 includes transistors Q5, Q6, and receives an input alternating current (AC) voltage signal vIF, so as to generate transconductance stage AC current signals iIF.
The resistor R1 has a first terminal receiving a direct current (DC) voltage VDD, and a second terminal coupled to the first terminals of the transistors Q1, Q3. The resistor R2 has a first terminal receiving the DC voltage VDD, and a second terminal coupled to the first terminals of the transistors Q2, Q4. A mixed frequency voltage signal vRF is outputted at the second terminals of the resistors R1, R2.
Since the voltage signal LO is a differential signal, when the transistors Q1, Q2 conduct, the transistors Q3, Q4 are cut-off, and vice versa.
Through operation of the Gilbert cell mixer, the mixed frequency voltage signal vRF will have a frequency fRF equal to a sum of a frequency fIF of the input AC voltage signal vIF and a frequency of the local oscillator voltage signal fLO (i.e., fRF=fIF+fLO)
The conventional Gilbert cell mixer has the following drawbacks:
1. In direct current (DC) analysis, DC currents IDC entirely flow through the resistors R1, R2, resulting in large power consumption.
2. The DC currents IDC cause a large voltage drop between the two terminals of the resistors R1, R2, thereby reducing conversion gain of the Gilbert cell mixer. If amplitude of the input AC voltage signal vIF is increased for a higher conversion gain, linearity of the Gilbert cell mixer may be adversely affected.